To upgrade a certain account you're managing in ManyChat to the Pro plan, you need to click blue 'Upgrade to Pro' button on the left panel:
You will be presented with this window then:
Click 'Upgrade To Pro', then provide your billing credentials, and hit the 'Pay' button here:
That's it — the page should become Pro right away!
PS: If anything goes wrong, please check out our article about Billing issues.